We have investigated the electronic structures, magnetic hyperfine and nuclear quadrupole interactions in a series of heme compounds with different fifth ligands (fluoride, chloride, bromide, hydroxide and cyanide) and in a model compound for metmyglobin, involving water and imidazole as fifth and sixth ligands. The hyperfine constants of the Fe57m and N14 nuclei per unit spin agree within 4 and 10 per cent respectively for the first four heme derivatives, the cyan derivative having rather different hyperfine constants. For Fe 57m in the chloride derivative excellent agreement is found with the experimental Mossbauer hyperfine field in hemin. For N14 in the fluoride, chloride and bromide derivatives, the closeness of the predicted hyperfine constants is borne out experimentally, with the theoretical values being all about 60% of experiment. The Fe 57m hyperfine field in the metmyoglobin model compound agrees very well with experiment and is close to hemin. The N 14 hyperfine constant for the porphyrin nitrogens is found to be nearly the same as in the heme compounds and about 3 Mhz smaller than for the imidazole nitrogen bonded to iron, in agreement with the experimental direction in metmyglobin. These results indicate that the electronic structures in the molecules are reasonably well described by theory and that the electronic distribution at the imidazole nitrogen would be more sensitive than those at the iron and prophyrin nitrogens, as to the influence of changes in atoms and groups attached to iron and imidazole or other causes. Work is in progress in other related systems and on other electronic properties to further test these conclusions. BIBLIOGRAPHIC REFERENCES: "Core-Electron Contribution for Fe57m Nuclear Quadrupole Interaction in Hemin", (Jane C. Chang, Dennis Ikenberry and T. P. Das), Theo. Chim. Acta 35, 361 (1974). "Theory of Exchange Splitting of Core Levels in X-ray Photoemission Spectroscopy of Hemin", (Jane C. Chang, Y. M. Kim and T. P. Das), Brief report submitted for presentation at APS Philadelphia Meeting, March 1974, J. Chem. Phys. 62, 4350 (1975).